Prior to the present invention, cyclic poly(siloxane)s were generally made by hydrolyzing diorgano dihalosilanes which resulted in a mixture of cyclic poly(diorganosiloxane)s and linear poly(diorganosiloxane)s. The proportion of cyclic poly(siloxane)s to linear oligomers can be optimized by careful choice of the solvent and conditions of the hydrolysis. Although the hydrolysis method has been the preferred procedure for preparing cyclic poly(dimethylsiloxane), it has been found less suitable for the synthesis of poly(methylhydrogensiloxane).
The preparation of cyclic poly(dimethylsiloxane)s by the thermolysis of poly(dimethylsiloxane)s in the presence of sodium hydroxide has been described by Hunter et al., Journal of the American Chemical Society, 68, 667, 1946. Additional methods for making cyclic poly(dimethylsiloxane)s from poly(dimethylsiloxane) is shown by Gustavson et al., Journal of Organometallic Chemistry. 238, 87 (1982) utilizing a transition metal complex, such as a rhodium or iridium-containing complex.
A further method for making cyclic poly(siloxane)s including cyclic poly(siloxane)s having hydrogen functionality is shown by U.S. Pat. No. 3,714,213, Miller et al. which employs an acid treated clay. Although Miller et al.'s method has been found to provide hydrogen containing cyclic poly(siloxane)s at satisfactory yields, it allows for the presence of large amounts of hydrogen-functional poly(siloxane) at high temperature during the formation of the cyclic poly(siloxane). This reduces the likelihood that the Miller et al. procedure can be commercially scaled up because a considerable risk of explosion is present. Another procedure for making cyclic hydrogen functional poly(siloxane)s is shown by Endo, Japan Kokai AP 52/69500, June 9, 1977; CA Vol. 87, 1977, page 28: 118455b. The Endo procedure is also dangerous, as it allows for the build up of large amounts of linear poly(methylhydrogensiloxane) at elevated temperatures. Halm et al., U.S. Pat. No. 4,764,631, provides a procedure for reforming cyclic poly(siloxane)s utilizing an alkali metal compound. This method would be unsuitable for making cyclic poly(methylhydrogen siloxane), as it requires an alkali metal compound.
It would be desirable, therefore, to provide a safe procedure for making cyclic poly(siloxane)s having chemically combined diorganosiloxy units, or organohydrogen siloxy units, and mixtures thereof which could be commercially scaled up without any danger of explosion. In addition, it also would be desirable to make homopolymers or copolymers of diorganosiloxy units and organohydrogen siloxy units in cyclic form at satisfactory yields in a simple and direct manner.
The present invention is based on the discovery that if substantially linear organosiloxane, including silicone materials having organohydrogen siloxy units is added dropwise onto a heated fixed catalyst bed, there is produced volatile cyclic poly(organosiloxane) upon contact. It has been further found that if the vaporous cyclic poly(organosiloxane) is immediately condensed at a point remote from the catalyst bed, build-up of vaporous cyclic poly(organosiloxane) in contact with such hot surface can be avoided.